STS_Site

The STS_Site property is used to find site collections, or actually just find the top level sites of site collections. To list all of the top level sites that you have permissions to see, search for "ContentClass:STS_Site". (See warning below!)
My test user Sam found 8 site collections and my administrator account found 38.

STS_Web

The STS_Web property is used to find subsites. To list all of the subsites that you have permissions to see, search for "ContentClass:STS_Site". (See warning below!)
Sam found 9 subsites while my administrator account found 158.

Find all sites you have access to…

Just combine both searches with an "OR" to show all of the sites you have access to. (Remember that Boolean operators like AND, OR and NOT must be in UPPER case.)

Adding Keywords

A little tip: If you add keyword to your STS_Web and STS_Site queries you will not be searching content in the site. You instead will be searching for that keyword as a property of the home page or content that might be displayed on the home page. The following search will only return results when "plane" in somewhere on the home page of the site:

That's doable, but not with search. You can write a PowerShell script to do this, but it is a bit busy as it has to check each site for UserA, each SharePoint group that hass access to any content in the site for UserA and each Active Directory group that hass access to any content in the site for UserA.

Are you working with an on-prem SharePoint farm or Office 365? Are you a farm or tenant administrator?

It is hosted, not on-prem. I'm not the adminstrator. She isn't really a sharepoint expert. Neither am I:-(I've just been tasked to look into radically expanding our use of sharepoint for collaboration with external users, and being able to manage and specifically remove people's access if the relationship ends naturally, or is terminated.

Up until now the approach has to give them AD accounts in our system, which we can delete. However that gets unwieldy as we expand to more external users. It also doesn't allow fine grained control or a view into what they can see. Its all or nothing, big hammer approach.